I. Field of the Invention
The present invention relates to apparatus used to stimulate body tissue, including cardiac pacemakers.
II. Background of the Invention
Packmakers are well-known to comprise pulse-generating circuits when periodically deliver electrical pulse to the proximal end of an electrode. The distal end of the electrode contains an exposed conductive tip and is placed adjacent muscle tissue of the heart. The electrode thereby operates to deliver the pulses from the pacemaker to the heart to stimulate the heart.
All humans, including implantees of pacemakers, are subject to being exposed to externally high voltages. For example, if ventricular fibrillation, an abnormal rhythm of the heartbeat, should occur, one means of securing return to a normal rhythm is to apply a pair of conductive paddles to the exterior of the body and place a high voltage across the paddles. The resultant current flow through the heart often reverts the heart to a normal rhythm. This process is called defibrillation. Defibrillation can even be used to start a stopped heart.
Cardioversion is a more general term of the process of exposing the heart to a high current to correct arrhythmias.
Diathermy is another process by which high currents are introduced to the human body. Diathermy is the process wherein an electrical current is concentrated at a point on a body to cut tissue and arrest bleeding. In each of these situations, defibrillation, cardioversion, and diathermy, when a pacemaker is inside the patient, a current goes through the pacemaker and associated electrode because the pacemaker and associated electrode are more conductive than the surrounding body tissue. The known prior art recognizes that such a current through the pacemaker and associated electrode could cause an adverse effect on the operation of the pacemaker. Accordingly, the known prior art places one or more Zener diodes or similar devices between the output and indifferent terminals or ground of the pacemaker. This arrangement limits the amount of current and voltage surge which may be introduced from the electrode into the pacemaker upon application of high voltages to the exterior of the patient's body.
While such prior art arrangements protect the circuitry of the pacemaker, there is no limitation on the current flow which is permitted along the electrode and in particular which is permitted to flow between the distal tip of the electrode and the heart tissues adjacent that tip. This current flow has been found to cause considerable damage. First, the current may damage the heart itself. Second, the current can increase the pacing threshold of the heart, which is the amount of electrical signal required from the pacemaker to stimulate the heart. Specifically, this current can cause fibrosis of the tissue adjacent the electrode tip and thereby increase the distance between the electrode tip and excitable heart tissue.
It is, therefore, an object of the present invention to protect the heart tissue of a pacemaker implantee from damage upon application of high voltages to the user's body as may occur with defibrillation, cardioversion, or diathermy.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.